Field of the Invention
The present invention relates to an image capturing apparatus that captures a tomographic image of an object to be inspected by optical coherence tomography and a noise removal method and noise removal program for tomographic images.
Description of the Related Art
Image capturing apparatuses based on optical coherence tomography (OCT) utilizing interference of low coherent light are well known. (Such an apparatus will be hereinafter referred to as an OCT apparatus). The OCT apparatus can generate a tomographic image of an object to be inspected with high resolution.
The OCT apparatus splits light from a light source into measurement light and reference light by means of a beam splitter or the like. The measurement light is delivered to an object to be inspected such as an eye, and the measurement light is reflected and diffused by the object to be inspected. The resulting return light from the object to be inspected is combined with the reference light having passed through a reference mirror, and guided to a detector. The return light has information about the layers in the object to be inspected in the irradiation direction. Interference light of the return light and the reference light is detected by the detector. A tomographic image of the object to be inspected is acquired by analyzing the interference signal thus obtained.
A. F. Fercher, C. k. Hitzenberger, G. Kamp, S. Y. El-Zaiat, Opt, Commun. 117, 43-48, (1995) disclose a Fourier domain OCT apparatus (which will be hereinafter referred to as FD-OCT apparatus), which obtains wavelength spectrum with a fixed reference mirror and performs tomographic measurement by Fourier-transforming the wavelength spectrum. The FD-OCT apparatuses include SD-OCT (spectral domain-OCT) apparatuses using a spectrometer and SS-OCT (swept source-OCT) apparatuses that sweep the wavelength of the light source.
In tomographic imaging of an object to be inspected with an OCT apparatus, the spectral intensity distribution of the reference light is also Fourier-transformed. Consequently, intensity information of each frequency component appears as noise in the image. Moreover, in cases where multiple reflection occurs in the optical system, frequency components resulting from interference with multiple-reflected light are generated. In this case also, noise resulting from such frequency components appears in the image.
According to a known method of removing such noise, information about the spectral intensity distribution of the light source is obtained in advance by measurement to remove noise (see S. Moon, S. W. Lee, Z. Chen, Optics Express, Vol. 18, No. 23, 24395-24404, (2010)). However, in cases where the intensity of light emitted from the light source is unstable or where the light quantity changes due to the transmittance of optical components, vibration of optical components, temperature changes, and/or other factors, the noise intensity also changes, so that residual noise remains unremoved.
An apparatus disclosed in Japanese Patent Application Laid-Open No. 2011-242177 is intended to remove such noise. This apparatus is provided with a system for detecting a reference light signal for correction separately from a system for detecting a reference signal obtained by interference of reference light and measurement light. These systems detect the interference signal and the reference light signal separately from each other, so that the interference signal is corrected by the reference light signal. By this technique, the interference signal and the correction signal are measured concurrently. This enables removal of noise even if the light quantity of the reference light changes (or the light quantity of the interference signal changes).
The apparatus disclosed in Japanese Patent Application Laid-Open No. 2011-242177 is required to have an additional optical system and an additional detector for the purpose of obtaining the reference light signal for correction. Moreover, it is necessary that the light quantity of the interference light and the light quantity of the reference light signal for correction be adjusted to be balanced among the detectors. Therefore, the construction of the apparatus and the adjustment process are complex.